Atomic clocks are considered the most accurate time and frequency standards known to date, and are used as primary standards for international time distribution services, for example, in applications such as communications, finance, navigation and location determination systems. An atomic clock may use an electron transition as a frequency standard for its timekeeping element. The frequency standard can be, for example, a frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum that, for example, electrons in atoms may emit when they change energy levels. Early atomic clocks were based on masers at room temperature. Currently, the most accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity.
Precision timing may be essential for inertial navigation systems operating in the absence of GPS, as well as for GPS receivers operating in areas with noisy or intermittent GPS readings. Generating timing locally on a receiving platform may reduce or eliminate the reliance on GPS signals for timing. While clocks have been used to generate timing locally, it is understood that previous clocks have been either far too large, heavy, and/or power-hungry to use on a receiving platform or far too inaccurate or unstable to be usable for many demanding applications. Generating timing locally on a receiving platform may reduce or eliminate the reliance on GPS signals for timing.
Low size, weight, and power (SWaP) microwave neutral atom clocks based on small vapor cells have been developed and are called chip scale atomic clocks (CSAC). CSAC devices, such as compact cesium (Cs) and Rubidium (Rb) clocks, have instability (at 1 sec integration duration) of 3.5×10−10, long term aging of 9×10−10/month, and maximum frequency change of 5×10−10 over an operating temperature range of −10° C. to +35° C. CSAC devices may have inaccuracy of 10−10. While low SWaP, the stability and accuracy of CSAC devices may be 10−4 that of some embodiments herein and 1×10−8 that of laboratory-grade optical clocks. Moreover, CSAC devices may rely on vapor cells that are very sensitive to environmental temperature and may display aging. CSAC devices may also require extensive (e.g., 6-12 hours) calibration after turn-on and may be limited to mission durations of 3-6 hours due to temperature sensitivity and aging. On the other hand, higher precision Cs and Rb clocks such as those commercially available may provide improved accuracy and stability (e.g., 5×10−13 inaccuracy and 5×10−12 instability), but at the cost of significantly higher SWaP (e.g., these devices may have a volume of approximately 10 liters, while CSAC devices may have a volume of approximately 20 cubic centimeters).